The objectives of this project are to determine the influence of microwave radiation on cardiac tissue in vitro and in vivo. A method for exposing isolated rat atria to microwave radiation has been developed. The data suggest that 2.45 GHZ CW microwave radiation of 2 or 10 mW/g has no overt effect on the rate or force of contraction of isolated atria. However the response of atria to drugs was influenced by microwave exposure. Specifically the dose response curve for propranolol was shifted to the right during 10 and 100 mW/g exposure and the ability of propranolol to inhibit isoproteronol actions on rat atria was diminished by these exposure levels. The influence of microwave radiation on the response of isolated atria to other cardiotonic drugs are being undertaken. Also, certain biochemical and physiological parameters, which are indicative of cardiac integrity, have been measured in unanesthetized rats during whole body ventral exposure to 2450 MHz CW microwaves. Preliminary data suggest microwave exposure of 10 mW/cm squared for 6 hr has no effect on mean arterial blood pressure or colonic temperature. However, there was a microwave induced bradycardia which was exhibited after 30 min of microwave exposure at 10 mW/cm squared and persisted throughout the remainder of the 4 hr exposure period. None of the biochemical or hematologic indices examined were influenced by this exposure level.